This invention relates to the production of ethylene from a predominantly C3-5-paraffinic feedstock.
Ethylene is a commonly used feedstock and is used to form a variety of end and intermediate products, including ethylene oxide, ethyl acetate, and ethylene polymers and copolymers. The polymers are used to form a variety of plastics, resins fibers, and the like. Ethylene is one of the leading petrochemicals in terms of production volume, sales value and number of derivatives. An estimated 76 million tons were produced in 1995 alone, and a roughly three percent increase per year is expected. It will be difficult to meet the needs for ethylene in the future with existing methodologies.
There are several commercial methods for generating ethylene. One method under development involves the conversion of methanol to olefins (see, for example, U.S. Pat. No. 4,499,327 to Kaiser). Ethylene has been prepared from oxygenated materials, for example, alcohols, by converting the oxygenated materials to olefins, and metathesizing olefins such as propylene to produce ethylene. See, for example, U.S. Pat. No. 5,990,360 to Barger et al. A limitation of this approach is that oxygenated materials are a relatively expensive feedstock for use in preparing ethylene.
The main commercial methods involve hydrocarbon pyrolysis, also known as steam cracking. This technique can use a number of different hydrocarbon feedstocks. When ethane is the feed, an ethane cracker can be used. This technique is the least capital intensive and provides the fewest by-products. When the feed includes a mixture of ethane and propane, a more capital-intensive ethane-propane (EP) cracker is needed. When the feed includes a mixture of ethane to pentane, a still more capital-intensive flexi-cracker is needed. Accordingly, it is desirable to provide feedstocks that are rich in ethane, and possibly include propane, but preferably which do not include appreciable amounts of butane, pentane or higher molecular weight paraffins.
An example of a process using a less desirable feed for the cracking reaction is shown in U.S. Pat. No. 5,026,935 to Leyshon et al. Leyshon et al. disclose a process for preparing ethylene from butanes or higher molecular weight feedstocks via cracking and metathesis to form ethylene and propylene. At least a portion of the propylene is metathesized to ethylene. The method is relatively expensive because it requires using relatively expensive cracking conditions, and because it makes the overall efficiency and yield relatively low.
It would be desirable to provide a process for producing ethylene that does not require using a Flexicracker, using oxygenated feedstocks, or cracking significant amounts of relatively high molecular weight (i.e., C4+) products. The present invention provides such a process.
In its broadest aspect, the present invention is directed to an integrated process for producing ethylene or a mixture of ethylene and propylene from a feedstock that includes C3-5 paraffins. The process involves obtaining an appropriate C3-5-containing feedstock and subjecting the paraffins to conditions of molecular redistribution. During the course of this reaction, the feedstock undergoes a series of reactions involving paraffin dehydrogenation to form olefins, olefin metathesis, and olefin rehydrogenation to form paraffins. The molecular redistribution reaction provides a product stream that includes C2 and C6+paraffins, in addition to C3-5 paraffins.
The product stream can be distilled to provide a first fraction rich in either ethane or a mixture of ethane and propane, depending on the type of distillation apparatus, a second fraction rich in unconverted C3-5 paraffins and a third fraction containing predominantly C6+paraffins. The C2 stream can be sent to an ethane cracker to provide ethylene, or, alternatively, the C2-3 fraction can be sent to an ethane/propane (EP) cracker to provide ethylene and propylene. The C3-5 fraction can be recycled through the molecular redistribution stage to provide additional ethane, propane and C6+paraffins.
The C6+fraction from the molecular redistribution step tends to have low ppb sulfur, and can be used, for example, as a solvent, or as a feedstock for reforming processes to form aromatic compounds, for example the AROMAX(trademark) process or platforming or rheniforming processes. The products may also optionally be isomerized to increase their octane value, and the isomerized products can be used in gasoline compositions.
Depending on the nature of the molecular redistribution chemistry, the feedstock may not be able to include appreciable amounts (i.e., amounts that would adversely affect the catalyst used for molecular redistribution) of hydrogen, olefins, alkynes, thiols, amines, water, air, oxygenates or cycloparaffins.
Methane does not participate in the reaction (it cannot be dehydrogenated to form an olefin) but does dilute the reactants and may be required to be removed from the feedstock to improve the throughput of the reaction. Because the molecular redistribution reaction proceeds toward a thermodynamic equilibrium, the presence of ethane in the feedstock should be minimized, as it will limit conversion of C3-5 paraffins to ethane.
Hydrogen, water, air, methane and ethane can be removed from feedstocks using conventional methodology, for example using demethanizer and deethanizer columns. Methods for removing sulfur, oxygenates and nitrogen compounds are well known, and generally involve hydrotreating the feedstock. Methods for removing cyclic compounds are also known in the art and generally involve adsorption and separation by molecular sieves.
Refinery waste gas predominantly includes hydrogen and C1-5 paraffins, but may include small amounts of olefins and alkynes, as well as heteroatom-containing impurities. Natural gas predominantly includes C1-5 paraffins, but may include sulfur and nitrogen impurities. Cracked gas feedstreams predominantly include hydrogen and C1-6 paraffins, olefins, alkynes and sulfur and nitrogen impurities. The hydrogen, methane, ethane and heteroatom-containing impurities from these feedstocks are removed, and any olefins and/or alkynes hydrogenated, before the feedstocks are used in the processes described herein. LPG, derived from petroleum refining, contains mostly propane and butanes, with small amounts of pentane, and does not need to be demethanized and/or deethanized, but may need to be treated to remove impurities such as cycloparaffins and sulfur and nitrogen compounds.